Industrialists have developed many techniques allowing the evolution of the fluids injected into porous media to be monitored.
Geochemical monitoring methods for geological CO2 storage sites, based on the study of volatile species, are for example known. Examples of such methods are described in patent applications FR-2,972,758 and FR-2,974,358.
These methods essentially apply for two compartments:
in the reservoir/saline aquifers where the main objective is to quantify the dissolved and precipitated CO2, and thus to establish a real mass balance,
in the aquifers overlying the cap rock, where the main objective is to diagnose a leak as early as possible.
To implement these methods, it is thus necessary to have a device for sampling fluids under pressure in a well drilled through a geological formation. Such a device is referred to as sampler.
Samplers referred to as FTS (Flow-Through Sampler), allowing to obtain fluid samples from a well drilled through a geological formation, are known. Such a device is comprised of a sample chamber with a spring-loaded valve at each end. A latching mechanism connects the valves together and holds them open. Above the chamber, there is a clock to program the closing time and a triggering mechanism to release the valves. The lower end is provided with means allowing the fluid to enter. At the top, there is a rope socket for attaching a cable.
U.S. Pat. No. 5,945,611 discloses a device for sampling fluids under pressure in a pipeline, tube, duct, conduit or the like. This device comprises a plurality of pistons, a body having a common passageway, wherein said pistons are slidably mounted, a lateral inlet and a lateral outlet port located within said passageway and communicating with the pipeline, said inlet and outlet ports so arranged that the motion of the pistons can cover and uncover said inlet and outlet ports.
U.S. Pat. No. 5,896,926 discloses a device for in-situ sampling of groundwater under static conditions without disturbing the environment but comprising a packer to isolate the sampling system from the area located above, as well as an in-situ pumping system in said sampler for “sucking” the fluid into the sample chamber.
The French patent application whose application number is FR-12/03,329 (not published yet) discloses a sampling device comprising, on the one hand, a piston controlled by a spring immersed in an oil chamber for sampling the fluid and, on the other hand, a second piston for expelling the fluid upon transfer. The device is kept in open or closed position by the compressed spring housed in the oil-filled chamber. The oil contained in the spring chamber allows the decompression effect to be damped and smooth sampling to be achieved. The device enables recovery of the sampled fluid using the mechanical action of a solid piston through a manual valve. This design allows to avoid mercury systems or piston fluid systems, and to recover all or part of the fluid under controlled pressure conditions. Furthermore, this design allows to avoid using a surge chamber and an oil chamber as in nearly all of the known samplers. Another advantage of this device is that it can be lowered in open position into the underground medium, thus overcoming opening problems and allowing complete filling of the sample chamber. However, the device described in this patent application can pose problems upon closing of the sampler due to the fluid compressibility likely to hinder return of the first piston.
The invention relates to a fluid sampling device comprising a sample chamber including a lower piston, an upper piston and an intermediate piston. The intermediate piston is moved so as to guarantee a substantially constant chamber volume when closing the chamber in order to avoid problems linked with the incompressibility of the sampled fluid.